1. Field of the Invention
This invention relates to pressure-sensitive adhesive compositions and adhesive tapes, specifically acrylic-based cellular pressure-sensitive adhesive membranes having improved internal strength, excellent conformability and good low-temperature performance.
2. Description of the Related Art
Acrylate pressure-sensitive adhesives are well known in the art. In U.S. Pat. No. Re 24,906 (Ulrich), alkyl acrylate copolymers are described which are predominately alkyl esters of acrylic acid having from 4 to 14 carbon atoms, and further comprise a minor amount (3-12%, preferably 4-8%) of a polar copolymerizable monomer such as acrylic acid. Such adhesives are widely popular as they are readily available and provide a good balance of tack, shear and peel properties on a variety of substrates at a relatively low cost.
These adhesives have been used for various automotive applications, e.g., attachment of decorative items to the painted surface. Automotive industry testing of adhesives typically subjects adhesives to a shock test, known in the industry as a "cold slam" test, at temperatures down to -45.degree. C. These conventional acrylate adhesives have difficulty performing well on such tests, especially at the lower temperatures on new high solids paints systems which are increasingly used in the automotive industry.
U.S. Pat. No. 4,181,752 (Martens et al) discloses a process for ultraviolet photopolymerization of alkyl acrylate esters and polar copolymerizable monomers to form the acrylate copolymer. Martens teaches that intensity and spectral distribution of the irradiation must be controlled in order to attain desirable cohesive strengths and peel resistance. The photopolymerization is preferably carried out in an inert atmosphere as oxygen tends to inhibit the reaction. Adhesive properties for tapes made via the Martens process are improved over those made by solution polymerization, but still do not perform adequately on the automotive testing at the lowest temperatures.
Additional patents disclose ultraviolet radiation of acrylate adhesives. U.S. Pat. No. 4,364,972 (Moon) discloses the use of N-vinylpyrrolidone as the polar copolymerizable monomer in the acrylate adhesive copolymer. High adhesion to automotive paints is disclosed but not exemplified. U.S. Pat. No. 4,391,687 (Vesley) discloses the use of specific chromophore-substituted-halomethyl-s-triazines as photoactive crosslinkers for acrylate copolymers. U.S. Pat. No. 4,599,265 (Esmay) discloses a readily peelable pressure-sensitive adhesive tape, the adhesive layer of which is highly crosslinked and low in polar monomer content. While properties vary, none of these adhesives show improvements at the lowest temperatures on the automotive testing.
U.S. Pat. No. 4,243,500 (Glennon) discloses a pressure-sensitive adhesive formed from a composition comprising at least one monofunctional unsaturated acrylate ester monomer, saturated tackifying resin polymers, non-crystallizing elastomeric material, and an initiator responsive to ultraviolet light or other penetrating radiation. Glennon specifies the use of UV light within a wavelength range of 1800-4000 Angstroms. The intensity of the lamps to which the adhesive is exposed is much higher than that of the lamps disclosed in Martens et al.
Glennon states that the elastomer may be any natural or synthetic elastomer which is soluble in the acrylate ester monomer and which is of a non-crystallizing amorphous nature. The elastomer is present in from about 4 parts to about 150 parts per hundred parts acrylate monomer. The Glennon adhesive also requires the use of from about 50 parts to about 250 parts of tackifying resin per hundred parts acrylate ester monomer in order to obtain a satisfactory adhesion level to metal and painted steel substrates. Such levels of tackifier increase the glass transition temperature of the polymer matrix which has a negative effect on low temperature flexibility, and act as chain transfer agents during the polymerization. Further, tackifiers tend to migrate to the surface of the adhesive upon aging, causing the adhesion to deteriorate. Tapes made with Glennon adhesives will not pass the automotive testing.
Canadian Patent No. 1,192,688 (Moser et al.) discloses an adhesive composition comprising an acrylic monomer yielding a polymer with a glass transition temperature between 0.degree. C. and 100.degree. C. and at least one dispersible core-shell polymer, the composition being a 100% reactive fluid adhesive. Such adhesives cannot be used as pressure-sensitive adhesives.
European Patent Application, Publication No. 238863, (Klingler et al.) discloses a radiation or heat cured elastomer which exhibits enhanced physical properties by virtue of its cure resulting in phase segregation. The elastomer is dissolved in a methacrylate monomer in the presence of a photoinitiator or thermal initiator. The composition may also contain up to 20% by weight of a reactive acrylic diluent. Operable block copolymers include styrene/elastomer block copolymers, polyurethane block copolymers and polyester/polyether block copolymers. Acrylates analogous to the useful methacrylates are disclosed to yield a transparent non-phase-segregated product which exhibits markedly inferior physical properties.
U.S. Pat. No. 4,126,504, (Wolinski et al.) discloses a two-part fast curing adhesive wherein the first part is made up of a thermoplastic non-reactive elastomeric polymer dissolved in a free-radical addition polymerization combination of an acrylic or methacrylic monomer, a copolymerizable monomer containing at least one free carboxylic acid group, and a non-activated free radical addition polymerization catalyst system. The second part comprises an activator system for the free radical catalyst system, and optional accelerator for such system. In one modification, the activator may be encapsulated into insoluble, rupturable microspheres to form a single liquid adhesive. The adhesives disclosed in Wolinski must be applied to one surface, and the activator applied to the other surface, or in the case of single liquid adhesive, a catalyst must still be added at the time of use. The adhesives are, therefore, not appropriate for use as pressure-sensitive adhesives.
U.S. Pat. No. 3,832,274, (Owston) discloses fast-curing structural adhesives comprising (a) from about 1 to about 30% of an elastomer polymer selected from poly(butadiene) homopolymer, copolymers of butadiene with at least one copolymerizable monomer such as styrene, acrylonitrile and methacrylonitrile, and a copolymer of butadiene selected from the group consisting of homopolymer and copolymer modified by inclusion of up to 5% of a functional monomer; (b) 25% to 85% of at least one polymerizable acrylic monomer selected from acrylates methacrylates, acrylonitrile and methacrylonitrile; (c) 0 to 50% of an ethylenically unsaturated non-acrylic monomer; (d) polymers having intrinsic viscosity in the range from about 0.1 to 1.3, derived from (b) or (c); (e) 5% to 20% of methacrylic acid; and (f) 0.04% to 4% of the reducing component of a redox catalyst polymerization system. The oxidizing component of such catalyst system must be added within fifteen minutes of cure time after the adhesive is in place. The adhesives are structural cements, and are not suitable for use as pressure-sensitive adhesives.
Foam or foam-like adhesives are also well known in the art. This type of adhesive is desirable for adhesion to rough surfaces, or in situations where the adhesive may be subject to compression.
U.S. Pat. No. 4,223,067, (Levens) discloses a glass microbubble-containing acrylic pressure-sensitive adhesive. The adhesive is disclosed to have good shear and peel, and to conform well to rough and uneven surfaces.
Because the microbubble-containing tape of the Levens patent has a foam-like appearance and character, it is sometimes called a "foam-like" tape even though its pressure-sensitive adhesive layer is substantially free of voids except for the hollow spaces within the microbubbles. The Levens patent in turn teaches that, where it is desired to adhere the foam-like tape "to a surface to which its pressure-sensitive adhesive layer would not form a strong bond, it may be desirable to apply to one or both of its faces of its microbubble-filled adhesive layer a layer of unfilled pressure-sensitive adhesive which is especially selected for adhesion to that surface" (col. 4, lines 9-15). Such microbubble-free surface layers can also provide substantially increased cohesive strength, especially at high temperatures. Microbubble-free surface layers can have different adhesive properties, each selected for good adhesion to a certain surface. Because the application of those added layers substantially increase the cost of the foam-like tape, less expensive foam-backed tapes have dominated the market for uses requiring immediate adhesion to rough or uneven surfaces.
U.S. Pat. No. 3,565,247 (Brochman) concerns a pressure-sensitive adhesive tape, the adhesive layer of which is a foam. To make such a tape, a blowing agent and a nucleating-reinforcing agent such as fumed silica are blended into a solution of a pressure-sensitive adhesive. After this blend is coated, it is heated to evaporate the solvent. The temperature is then increased to that necessary to decompose the blowing agent, causing it to release a gas which forms minute microcells throughout the dried pressure-sensitive adhesive layer.
U.S. Pat. No. 4,855,170 (Darvell et al.) discloses a pressure sensitive tape construction containing resilient polymeric microspheres.
U.S. Pat. No. 4,415,615, (Esmay, Johnson, Vesley), discloses a cellular pressure-sensitive adhesive comprising 15% to 85% voids that does not collapse after being briefly compressed, has good adhesion on contact with rough surfaces, and conformability at sub-freezing temperatures, i.e., temperatures down to about 40.degree. C.
U.S. Pat. No. 4,767,793 (Schisler et al.) discloses a process of producing a foamed polymer by mechanically frothing a radiation-curable composition and curing the foam composition. Foam laminates for use as printing blankets are disclosed to contain various types of polymers, including a layer containing elastomers. The laminate may have an adhesive layer laminated thereto. Radiation-cured pressure-sensitive adhesives are not disclosed.
The adhesive layer of Esmay is made by frothing an acrylate monomer mixture composition which is photopolymerizable to a pressure-sensitive adhesive state and coating that froth onto a backing, and photopolymerizing to a pressure-sensitive adhesive having a cellular structure. The cellular adhesive layer of the Esmay tape comprises at least 15% voids by volume, and recovers substantially its original thickness after compression.
However, the action of frothing the acrylic polymer matrix reduces the internal strength of these cellular pressure-sensitive adhesive membranes relative to nonfrothed samples.
U.S. Pat. No. 4,378,278, (Allaway et al.) discloses a polymeric foam made by mixing a liquid multi-functional acrylate monomer and a liquid nonionic fluoronated alkyl-ester surfactant, foaming the surfactant and monomer and exposing the mixture to electron beam radiation.
The inventors of the present invention have now discovered that certain ultraviolet-radiation polymerized phase-separated cellular pressure-sensitive adhesive membranes comprising an acrylic polymer containing an alkyl acrylate monomer and a polar monomer, and a saturated hydrocarbon elastomer, or blend of hydrocarbon elastomers, each elastomer having at least one segment with a glass transition temperature lower than that of the acrylic polymer, exhibit improved internal strength of the membrane while exhibiting excellent conformability and adhesion properties.